The present invention concerns an optical measuring device for detecting the coloring of test fields of a test strip, which is wetted with a liquid for detecting substances in said liquid, whereupon the reflectivity of the test fields changes depending on the concentration of the substances to be detected, comprising                a measuring plane wherein said test strip is to be placed,        an illumination device for illuminating the measuring plane,        a planar image sensor,        an optical system for imaging said measuring plane onto the image sensor, and        an electronic evaluation unit for evaluation of the signals detected by said image sensor.        
Optical measuring devices of this kind are e.g. known as part of test strip analysis apparatus, which are used for examining urine or blood test strips in doctor's practices, hospitals or medical laboratories. The test strips to be analyzed often comprise a plurality of test fields, each of which serves to detect another substance. In such measuring devices, the test strip is illuminated as a whole and imaged onto the image sensor. From the image obtained, the electronic evaluation unit may then determine the coloring of all test fields across the test strip. Thus, it is not necessary to scan the individual test fields and the measurement of the test strip may be conducted in a shorter time, wherein the structure of the measuring device is simpler.
For detecting the coloring of a test field, its reflectivity for light of three different wavelengths has to be determined. In a conventional measuring device, the test strip is illuminated with white light and imaged onto an image sensor comprising sensor elements, each of which is sensitive to one of three wavelengths (colors). The light-sensitive elements are arranged in a planar matrix whose lines each comprise sensor elements sensitive to the light of the same color. Thus, there are three types of such color lines comprised in the image sensor, the order of adjacent color lines being such that one color line of the first type is followed by one of the second type and one of the second type is followed by one of the third type and one of the third type is followed by one of the first type.
Thus, three images are effectively detected by the image sensor sequentially, each time one of the entirety of color lines of one type, which represent the intensity of three spectral components of the same colored picture. From the intensity of the spectral components in every image point, basically a colored image may be calculated.
However, a problem is posed by the fact that these three images do not really represent the spectral components of the same image points since the color lines are offset from each other. If now these three images are taken for color evaluation, a systematic error occurs due to the finite width of the color lines.